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United States Patent |
5,325,059
|
Doty
|
June 28, 1994
|
DOR NMR sample spinner
Abstract
An NMR DOR sample spinner includes an inner-rotor that is driven by a
radial-inflow microturbine at each end, supported radially by anti-whirl
air bearings, and supported axially by thrust bearings at each end
thereof. The DOR spinner further comprises an outer-rotor that houses the
inner-rotor bearing and drive nozzles such that the inner-rotor axis is
inclined with respect to the axis of the outer-rotor at an angle of
typically 30.56.degree.. The outer-rotor is driven by a radial-inflow
microturbine at each end, supported radially on anti-whirl air bearings,
and supported axially by thrust bearings at each end thereof. The
outer-rotor further comprises ceramic bearing races at each end that hold
the sample drive nozzles in place. The outer microturbines are screwed to
the outer-rotor and hold the bearing races in place. Drive and bearing gas
for the inner-rotor is supplied through slip-fit, precision axial tubes at
each end of the outer-rotor. The rf magnetization coil surrounds the
central region of the outer-rotor, and rf shield rings limit the axial
extent of the rf magnetic field.
Inventors:
|
Doty; F. David (Columbia, SC)
|
Assignee:
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Doty Scientific Inc. (Columbia, SC)
|
Appl. No.:
|
858235 |
Filed:
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March 26, 1992 |
Current U.S. Class: |
324/321 |
Intern'l Class: |
G01V 003/00 |
Field of Search: |
324/321,318,300,307
|
References Cited
U.S. Patent Documents
4254373 | Mar., 1978 | Lippmaa et al. | 324/321.
|
4456882 | Jan., 1982 | Doty | 324/321.
|
4557611 | Aug., 1984 | Sixsmith | 384/124.
|
4739270 | Jun., 1986 | Daugaard et al. | 324/321.
|
4899111 | Aug., 1988 | Pines et al. | 324/321.
|
4968938 | Jun., 1989 | Pines et al. | 324/321.
|
4968939 | Jun., 1989 | Pines et al. | 324/321.
|
5159271 | Oct., 1992 | Llor | 324/321.
|
Other References
"NMR Experiments With a New Double Rotor", Journal of Magnetic Resonance,
89, 297-308 (1990) by Wu et al. (no month).
"High Efficiency Microturbine Technology", Proceedings, IECEC-91, Aug. 1991
by Doty et al.
"A Refrigeration System Incorporating A Low-Capacity, High-Speed,
Gas-Bearing Supported Expansion Turbine", Advances in Cryogenic
Engineering, vol. 8, 1962 by Mann et al. (no month).
"Double Rotor for Solid-State NMR" by Samoson et al. Rev. Sci. Instrum. 60
(10), Oct. 1989.
|
Primary Examiner: Arana; Louis
Attorney, Agent or Firm: Oppedahl & Larson
Claims
I claim:
1. A DOR NMR sample spinner for simultaneous rotation of a sample about
second and first axes in an external magnetic field comprising:
an outer-rotor having a second axis of static balance, said outer-rotor
being asymmetric about said second axis, said outer-rotor containing an
inner-rotor;
an outer stator including gas bearings for support of said outer-rotor,
drive nozzles to effect rotation of said outer-rotor, a gas distribution
manifold, and an rf magnetization coil surrounding said outer-rotor;
said outer-rotor comprising:
outer-rotor drive means at one end of said outer-rotor;
a cylindrical outer-rotor body having a rotational axis coincident with
said second axis and having a round through-hole defining a first axis
which intersects said second axis at an acute angle;
an inner bearing sleeve in said through-hole and two inner-rotor drive
stator caps at respective ends of said through-hole, each with an outside
diameter essentially equal to that of said through-hole;
an axial inlet port in said outer-rotor body for supplying pressurized gas
to said inner bearing sleeve and stator caps;
an outer bearing race disposed over the outside of each end of said
outer-rotor body so as to position and constrain said stator caps;
an outlet port in said outer-rotor body for venting gas from the central
region of said inner bearing sleeve;
dynamic balance means such that the dynamic balance axis of said
outer-rotor lies within 20 microns of said second axis when said
outer-rotor is assembled;
said inner-rotor being substantially reflectionally symmetric about a plane
containing its center of mass and perpendicular to its rotational axis,
said inner rotor being substantially rotationally symmetric about its
geometric axis, said geometric axis .being coincident with said first axis
during operation;
said inner-rotor further characterized in that it includes microturbine
blades and two opposed thrust bearing annuli;
said inner stator caps including inner-rotor drive nozzles to cooperate
with said microturbine blades, and including thrust bearing annular lands
to cooperate with said bearing annuli for rotation and centering of said
inner-rotor, such that the center of mass of said inner-rotor lies on the
rotational axis of said outer-rotor.
2. The DOR spinner of claim 1 further characterized in that the outer rotor
further comprises two symmetrically located partial-admittance
radial-inflow microturbines for rotation of said inner rotor.
3. The DOR spinner of claim 1 further characterized in that said
outer-rotor drive means is secured to said body by means of external,
axial threads on said body.
4. The DOR spinner of claim 1 in which said outer-rotor drive means
includes a radial-inflow microturbine blisk.
5. The DOR spinner of claim 1 in which said outer-rotor is symmetric with
respect to a rotation of 180 degrees in a plane containing the first and
second axes.
6. The DOR spinner of claim 1 in which the acute angle is 30.56 degrees.
7. The DOR spinner of claim 1 in which said inner bearing sleeve includes
gas bearing orifices inclined so as to impede the rotation of said
inner-rotor.
8. The DOR spinner of claim 1 in which said inner bearing sleeve contains
two circumferential rows of bearing holes near each end of said rotor.
9. The DOR spinner of claim 1 in which said outer stator further comprises
rf shield-rings a short distance beyond each end of said rf magnetization
coil.
10. The DOR spinner of claim 1 in which said inner-rotor comprises an
externally threaded sample cell and two inner-rotor turbine caps secured
to each end of said cell and maintained in precision colinear alignment by
matching shoulders.
11. The DOR spinner of claim 1 in which said inner rotor comprises two
short cylinders, each cylinder closed at one end, the cylinders secured
together by matching threads at their respective open ends, and maintained
in precision colinear alignment by matching shoulders.
12. The DOR spinner of claim 1 in which the mean radial clearance between
said bearing race and said outer rotor body is positive and less than 8
microns.
13. The DOR spinner of claim 1 in which said outer bearing race is made
from partially stabilized zirconia.
14. The DOR spinner of claim 1 in which said inner-rotor is made from a
composite containing at least 10% but not more than 20% carbon fiber.
15. The DOR spinner of claim 1 in which said inner-rotor stator caps are
made from a composite that includes at least 5% PTFE.
16. The DOR spinner of claim 1 in which said outer-rotor body is made from
a composite containing at least 6% but not more than 12% carbon fiber and
more than 5% but less than 25% quartz fiber.
17. The DOR spinner of claim 1 in which said outer bearing race and said
outer-rotor drive means are joined to form a drive-bearing cap.
18. The DOR spinner of claim 10 in which each said turbine cap includes an
integral number of precision internal threads and one of each said thrust
bearing annuli.
19. The DOR spinner of claim 10 in which said sample cell is made from
partially stabilized zirconia and has its outside diameter reduced near
its center compared to its outside diameter near its ends.
20. The DOR spinner of claim 2 further characterized in that the
radial-inflow microturbines are of reduced diameter whereby the bearing
annuli are at the full diameter of the inner rotor.
21. An outer-rotor for holding an inner-rotor for use with an DOR NMR
sample spinner for simultaneous rotation of a sample about second and
first axes in an external magnetic field, said outer-rotor having a second
axis of static balance, said outer-rotor being asymmetric about said
second axis, said outer-rotor comprising:
outer-rotor drive means at one end of said outer-rotor;
a cylindrical outer-rotor body having a rotational axis coincident with
said second axis and having a round through-hole defining a first axis
which intersects said second axis at an acute angle;
an inner bearing sleeve and two inner-rotor drive stator caps, each with an
outside diameter essentially equal to that of said through-hole;
an axial inlet port in said outer-rotor body for supplying pressurized gas
to said inner bearing sleeve and stator caps;
an outer bearing race over the outside of each end of said outer-rotor body
so as to position and constrain said stator caps;
an outlet port in said outer-rotor body for venting gas from the central
region of said inner bearing sleeve; and
dynamic balance means such that the dynamic balance axis of said
outer-rotor lies within 20 microns of said second axis when said
outer-rotor is assembled.
22. The outer-rotor of claim 21 further characterized in that the outer
rotor further comprises two symmetrically located partial-admittance
radial-inflow microturbines for rotation of said inner rotor.
23. The outer-rotor of claim 21 further characterized in that said
outer-rotor drive means is secured to said body by means of external,
axial threads on said body.
24. The outer-rotor of claim 21 in which said outer-rotor drive means
includes a radial-inflow microturbine blisk.
25. The outer-rotor of claim 21 in which said outer-rotor is symmetric with
respect to a rotation of 180 degrees in a plane containing the first and
second axes.
26. The outer-rotor of claim 21 in which the acute angle is 30.56 degrees.
27. The outer-rotor of claim 21 in which said inner bearing sleeve includes
gas bearing orifices inclined so as to impede the rotation of said
inner-rotor.
28. The outer-rotor of claim 21 in which said inner bearing sleeve contains
two circumferential rows of bearing holes near each end of said rotor.
29. The outer-rotor of claim 21 in which the mean radial clearance between
said bearing race and said outer rotor body is positive and less than 8
microns.
30. The outer-rotor of claim 21 in which said outer bearing race is made
from partially stabilized zirconia.
31. The outer-rotor of claim 21 in which said inner-rotor stator caps are
made from a composite that includes at least 5% PTFE.
32. The outer-rotor of claim 21 in which said outer-rotor body is made from
a composite containing at least 6% but not more than 12% carbon fiber and
more than 5% but less than 25% quartz fiber.
33. The outer-rotor of claim 21 in which said outer bearing race and said
outer-rotor drive means are joined to form a drive-bearing cap.
34. The outer-rotor of claim 22 further characterized in that the
radial-inflow microturbines are of reduced diameter whereby the bearing
annuli are at the full diameter of the inner rotor.
Description
FIELD OF THE INVENTION
The field of this invention is the measurement of nuclear magnetic
resonance (NMR) for the purpose of determining molecular or microscopic
structure, and, more particularly, the high resolution NMR measurement of
polycrystalline and/or amorphous solids having quadrupolar or second-order
interactions using high-speed spinning about two intersecting axes.
BACKGROUND OF THE INVENTION
High-speed sample spinning about an axis, inclined with respect to the
external magnetic field at the angle at which the second Legendre
polynomial vanishes, has long been used to average out dipolar NMR
interactions for improved spectral resolution. This technique is referred
to as Magic Angle Spinning (MAS). In U.S. Pat. No. 4,456,882 I disclose a
technique for high speed sample spinning using cylindrical, ceramic sample
containers with press-fit plastic turbines. Other NMR MAS spinners are
disclosed in U.S. Pat. Nos. 4,254,373 and 4,739,270 and the references
cited therein. Co-pending U.S. patent application No. 07/607,521,633, now
U.S. Pat. No. 5,202, and PCT appl. no. PCT/US91/01225 disclose further
significant improvements in gas bearing whirl stabilization, microturbine
efficiency, and high-temperature operation.
Pines et al (U.S. Pat. Nos. 4,899,111, 4,968,938, and 4,968,939) have shown
that spinning simultaneously about two intersecting axes, specifically the
zeros of the second and fourth Legendre polynomials, should be effective
in improving the spectral resolution of quadrupolar NMR nuclides. The
above patents provide eloquent discussions of the theory. This technique
has been called Double Rotation or DOR. The DOR technique is expect, ed to
be most effective at high B.sub.0 --typically 7 T to 18 T--with
inner-rotor rotational frequencies above 7 kHz and outer-rotor rotation
above 1400 Hz.
Prior-art DOR spinner designs, however, have been largely ineffective.
Special cases have shown spectral enhancement, but more often the spectra
are actually degraded, compared to MAS techniques, owing to the following:
(1) the presence of a large number of intense, closely spaced sidebands;
(2) inability to achieve stable spinning over the periods of time required
for adequate signal averaging or sideband suppression techniques;
(3) poor filling factor and hence low sensitivity;
(4) large NMR background signals; and
(5) limited temperature range.
Moreover, prior art designs typically require many hours of tedious,
dynamic balancing for every sample before marginally stable spinning can
be achieved. Spinners are then typically not usable for more than several
hours before they have been irreparably damaged by wear.
The instant invention offers such substantial performance improvements in
each of the above areas as to make the DOR NMR experiment a viable NMR
technique. Spinning speeds are more than doubled, thereby greatly reducing
the sideband problem. Samples are quickly loaded and easily spun without
tedious dynamic balancing. Filling factor is increased. Stable spinning
may now be maintained indefinitely, and spinners may be reused hundreds of
times. The design permits the use of such materials as may be required to
minimize background signals for any nuclide. The instant design can be
made of ceramic materials for operation over a wide range of temperatures.
SUMMARY OF THE INVENTION
An NMR DOR sample spinner includes an inner-rotor that is driven by a
radial-inflow microturbine at each end, supported radially by anti-whirl
air bearings, and supported axially by thrust bearings at each end
thereof. The DOR spinner further comprises an outer-rotor that houses the
inner-rotor bearing and drive nozzles such that the inner-rotor axis, or
first axis, is inclined with respect to the axis of the outer-rotor, or
second axis, at an angle of 30.56.degree.. The outer-rotor is driven by a
radial-inflow microturbine at each end, supported radially on anti-whirl
air bearings, and supported axially by air thrust bearings at each end.
The outer-rotor further comprises ceramic bearing races at each end that
hold the inner drive nozzles in place. The outer microturbines are screwed
to the outer-rotor and hold the bearing races in place. Drive and bearing
gas for the inner-rotor is supplied through slipfit, precision axial tubes
at each end of the outer-rotor. The rf magnetization coil surrounds the
central region of the outer-rotor, and rf shield rings limit the axial
extent of the rf magnetic field. The rotating parts are preferably made
from low-conductivity, carbon-fiber-reinforced plastics or from partially
stabilized zirconia.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with respect to drawings, of which:
FIG. 1 is a longitudinal cross section of the DOR NMR spinner system;
FIG. 2a is a longitudinal cross section and FIG. 2b is an end view of a
preferred embodiment of the inner-rotor assembly;
FIG. 3a is a longitudinal cross section, FIG. 3b is an end view along the
second axis, and FIG. 3c is a partial view along the first axis of the
preferred embodiment of the outer-rotor assembly;
FIG. 4 is a longitudinal cross section of a second preferred embodiment of
the inner-rotor assembly; and
FIG. 5 is a longitudinal cross section of a third preferred embodiment of
the inner-rotor assembly.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a longitudinal cross section of the DOR spinner assembly
according to one embodiment of the instant invention. The sample to be
studied by the DOR NMR technique is loaded into a DOR inner-rotor assembly
in area 1 at the center of FIG. 1. The inner rotor 100 is within an outer
rotor 200.
The sample and inner rotor 100 are more easily observed in FIG. 2a, which
shows a first preferred embodiment for the abovementioned inner rotor. The
sample is located in area 1.
The inner rotor shown in cross section in FIG. 2a has an axis of rotational
symmetry extending from left to right in the figure, and the rotor is
symmetric from left to right about the vertical plane containing its
center-of-mass. Stated differently, the cross section that is shown in
FIG. 2a is substantially unaffected by the angle to which the rotor has
turned when the section is taken, and the left and right portions of the
rotor as shown in FIG. 2a are very nearly mirror images of each other.
FIG. 2b shows an end view of the inner rotor of FIG. 2a. Turbine blades 122
are shown.
The inner rotor of FIG. 2a rotates within an outer-rotor assembly shown in
FIG. 3a at a predetermined angle as detailed in FIG. 3a in a first
preferred embodiment. The center of mass of the inner-rotor lies on the
second axis of the outer-rotor but need not be coincident with the center
of mass of the outer-rotor.
The first preferred embodiment of the inner-rotor, shown in FIGS. 2a and
2b, is especially well suited to manufacture from composites. The
cylindrical sample cell 101 has externally threaded, shouldered regions at
each end to accept precision-fit inner turbine caps 102, 103. The
inner-rotor assembly of FIG. 2a, which includes the sample 1, the sample
cell 101, and the turbine caps 102, 103, has an axial moment of inertia
I.sub.A about its axis of symmetry (running from left to right in FIG.
2a), and a transverse moment of inertia I.sub.T about a line perpendicular
to its axis and intersecting its center of mass. The symmetry conditions
stated earlier dictate that I.sub.T will be independent of the azimuthal
angle chosen, that is to say, independent of the arbitrary angle at which
a cross section such as that of FIG. 2a is taken.
Wu et al. ("NMR Experiments with a New Double Rotor", J. of Magnetic
Resonance, vol 89, pp. 297-309 (1990)) present one method of deriving the
governing equations for stable gyroscopic precession. A more intuitive
approach is to consider balancing the centrifugal forces (hence, torques)
from I.sub.T of the inner-rotor with the forced-precession torque of
I.sub.A. The value of the latter approach is that it is more immediately
clear that only the net torque on the inner-rotor can be canceled.
Substantial, unbalanced centrifugal forces remain on every plane along the
length of the rotor (radially outward near the ends, inward near the
center) but the torques add to zero. Either approach shows that for the
desired inner-rotor inclination of 30.56.degree. and for the desired
rotational frequency ratio f.sub.r (between 4 and 7) of inner-rotor
rotation f.sub.1 (Hz) to outer-rotor rotation f.sub.2 (Hz), the total
moment ratio should be between 5 and 9:
5<I.sub.T /I.sub.A <9 (1)
More preferably, the desired ratio of I.sub.T to I.sub.A usually lies
between 6 and 7, and it should be achieved for sample densities .rho.
between 1 and 6 g/cc. A frequency ratio f.sub.r of 5 may be satisfied in
numerous ways. Two examples are:
(1) a solid cylinder of uniform density with length L and diameter d where
L/d=3.07;
(2) a pair of thin disks of uniform density with diameter d symmetrically
spaced apart 2L on an axis where L/d=0.89.
For rotational frequencies of 10 kHz, the appropriate outside diameter
d.sub.1 for the sample cell is approximately 5 mm. The requirement of
accommodating several different sample cells for various samples then
establishes a minimum practical outer-rotor diameter of approximately 14
mm.
Inner-rotor design optimization includes the following:
(1) maintaining the moment ratio between 6 and 7 over a wide range of
sample densities;
(2) achieving maximum practical sample volume;
(3) achieving dimensional tolerances after repeated assembly such that the
geometric axis of symmetry lies within 10 microns of the dynamic balance
axis;
(4) securing the turbine caps against the large, unbalanced centrifugal
forces present on the caps during stable precession;
(5) providing convenient sample loading and removal;
(6) obtaining operation over a wide temperature range;
(7) selecting cell and turbine materials that do not introduce NMR
background signals;
(8) allowing relatively easy manufacturability; and
(9) tolerating momentary instabilities without excessive wear.
Partially stabilized zirconia (PSZ, .rho. between 5.7 and 6.0, depending on
stabilizers) and high-strength silicon nitrides (SN, predominately
Si.sub.3 N.sub.4, .rho. between 3.18 and 3.24) have generally been the
materials of choice for MAS sample cells for the past three years. The
primary applications for DOR will involve studies of .sup.27 Al and
.sup.17 O. Other applications will include .sup.23 Na, .sup.14 N, .sup.11
B, and other quadrupolar nuclides. SN is generally not an acceptable
sample cell material because of the minor alumina content required for
densification during sintering.
PSZ would often be acceptable for the inner-rotor since the natural
abundance of .sup.17 O is only 0.4% and samples could be isotopically
enriched, but its high density results in much higher bearing load than
plastics or composites during instability. Several plastics are well
suited, especially polyetherketone (PEK), polyimides, and polyphthalamide
(PPA). Other wear-resistant, high-strength insulating materials may also
be used, but fiber-glass-reinforced materials are usually not suitable
because of the aluminum and boron content in the fibers. For satisfactory
dielectric properties, carbon fiber reinforcement must be kept below 20%
in the sample cell and below 12% in larger parts, compared to the 20% to
30% loading typically used for structural purposes. Kevlar reinforcement
is usable with thermoplastics such as nylon 6,6 that have low processing
temperatures (250.degree. C. compared to 400.degree. C. for PEK); and
quartz fibers, though weak compared to other choices, can be beneficial,
particularly in increasing the modulus. Polytetrafluoroethylene (PTFE) may
be added in amounts of about 5% to 15% for improved wear resistance. In
all cases, the composites must be oriented so that direction of major
anisotropy, the drawing or compression direction during processing, lies
along the axis of the finished part.
Returning to FIG. 2a, precision, ultra-fine, external threads 104, 105 on
the ends of the sample cell allow the inner turbine caps 102, 103 to be
secured against the typical axial forces of 20 to 100 N and permit
convenient access for sample loading and unloading. Right-hand threads are
used at one end, and left-hand threads are used at the other end to
prevent the caps from unscrewing during instability. Precision alignment
and centering of the sample cell 101 is accomplished by means of mating
shoulders 106, 107, 108, 109 beyond both ends of the two threaded regions.
Suitable, zero-taper (bottoming) threads, preferably with an integer
number of turns, may be cut into composites using diamond or carbide
tooling precisely enough to eliminate the need for individual dynamic
balancing of the empty sample cell. Matching, precision, internal threads
111, 112 are required in the turbine caps. For composites, the static
balance axis, dynamic balance axis, and geometric axis of the inner-rotor
must be coincident within 20 microns and preferably within 4 microns.
Higher precision is required for ceramics.
Prior-art MAS and DOR sample cells utilizing internal threads on the cell
and external threads on a solid plug become inconveniently clogged with
the sample. Moreover, prior-art solid plug caps with external threads
restrict sample volume, and prior-art hollow plug caps with external
threads must be packed uniformly before being screwed into the cell.
The outside of the cylindrical body of the internally threaded turbine caps
102, 103 functions as a gas bearing journal 115, 116 at each end of the
inner-rotor. Radial-inflow microturbine blades 121, 122 are cut into the
end of the turbine cap blisks 123, 124, which extend axially beyond the
thrust bearings by typically 1 to 2 mm. The external ends 125, 126 may be
hollow or solid, according to sample density and desired moment ratio.
Net axial and radial forces are developed on the inner-rotor during stable
precession when the center of mass of the packed inner-rotor does not lie
on the axis of the outer-rotor. Sample access at both ends of the
inner-rotor is beneficial in obtaining uniformity of the packed sample 1
in the axial direction to assure that the center-of-mass of the
inner-rotor coincides with its geometric center. High-capacity thrust
bearing annuli 117, 118 at both ends of the rotor are necessary to
accommodate residual axial nonuniformity in sample packing. A small
external chamfer 127, 128 is generally beneficial in manufacturing.
FIGS. 3a, 3b, and 3c provide more detail of the preferred embodiment of the
outer-rotor assembly which supports and spins the inner-rotor assembly of
FIG. 2a simultaneously about two axes, axis-1 (the "first axis") and
axis-2 (the "second axis"). Axis-2, within manufacturing tolerances, is
coincident with the aces of the cylinders defined by the external surface
of outer-rotor body 201 and the outer-rotor bearing races 202, 203 and is
coincident with the static balance axis of rotor body 201. A throughhole,
whose axis intersects axis-2 at 30.56.degree., precisely aligns an inner
bearing sleeve 210 and two inner-rotor drive stator caps 220, 230. Axis-1,
within manufacturing tolerances, is coincident with the geometric axis of
sample cell 101 during operation.
Thrust bearing annuli 117, 118 on the perimeter of the external end of each
turbine cap 102, 103, in cooperation with adjacent annular lands 221, 231
on the inner-rotor drive stator caps 220, 230 maintain precise centering
of the inner-rotor within the outer-rotor of FIG. 3a in a manner similar
to that disclosed in co-pending patent application PCT/US91/01225.
The rotational frequency of the inner-rotor may exceed the product of the
rotational frequency of the outer-rotor and f.sub.r by an amount dependent
primarily on the load capacity of the inner bearing and the mean density
of the inner-rotor. The load capacity of the radial bearing sleeve 210 may
be greatly increased by allowing the bearing gas to exhaust over the
central portion of the inner-rotor, as discussed in co-pending patent
application Ser. No. PCT/US91/01225, through central bearing exhaust or
outlet ports 253, 254. In the same co-pending patent application, we also
disclose that whirl instabilities may be controlled by injecting the
bearing gas with a tangential component opposed to that of the rotor
rotation. To do this, the gas bearing orifices are inclined so as to
impede the rotation of the inner-rotor. This technique also increases load
capacity by a substantial amount at high surface speeds.
Some additional increase in load capacity for the inner bearing sleeve 210
is possible by providing two circumferential rows of bearing holes at each
end of the inner-rotor such that the outer bearing row 211, 212 is
positioned a distance less than d.sub.1 /3 from the end of the
inner-rotor, and the inner bearing row 213, 214 is positioned a distance
less that d.sub.1 /2 from the outer row.
Axial inlet ports 251, 252 supply pressurized gas to inner bearing supply
grooves 215, 216 and inner drive ports 217, 218 for the radial-inflow
microturbine nozzles 221, 231. The inner turbines are of the
partial-admittance type, with an admittance angle .alpha. less than
230.degree.--typically four nozzles subtending an azimuthal angle of about
140.degree. around axis-1, with the two nozzle groups located on opposite
sides of axis-1. This permits maximum inner rotor length with simplified
drive gas ducts. The relationship between bearing orifices 212, nozzles
231, and turbine blades 122 (see FIG. 2a) is shown in cross section in
FIG. 3c.
The preferred material for the sleeve 210 and bearing races 202, 203 is
usually partially stabilized zirconia. Polyimide composites lubricated
with 5% to 10% teflon offer the advantage of lower density; hence,
tolerance limitations result in less dynamic imbalance of the outer-rotor
of FIG. 3a but wear resistance and dimensional stability are degraded
compared to zirconia.
Dimensional stability and strength of plastic composites may be enhances
with carbon fiber preferably of 6% or more, but graphite lubricant is to
be avoided as the total carbon content must not exceed 12% for acceptable
dielectric properties in the inner-bearing sleeve 210, inner stator caps
220, 230, outer-rotor body 201, and outer-rotor bearing races 202, 203.
Higher carbon fiber loading, preferably at least 10% and up to 20%, is
permissible in the severely stressed sample cell 101 since its volume is
very small.
Additional outer-rotor strength could be provided by preferably more than
5% and less than 25% quartz fiber.
The outer rotor assembly as pictured in FIGS. 3a and 3b has static balance
axis coincident with geometric axis, axis-2. That is, in a uniform
gravitational field, it will not tend to rotate from any rest position in
a frictionless bearing system coincident with its geometric axis unless
other external torques are applied. However, it clearly has a large
dynamic couple imbalance, except for certain improbable choices of
relative densities of the various parts.
High-speed rotation is possible only if the dynamic balance axis is also
made to coincide with the geometric axis. Methods of measuring and
correcting dynamic imbalance are well known, and the art is widely
practiced. For example, if rotor body 201 and stator caps 220, 230 have
similar density, dynamic balance could be achieved by drilling suitable
balancing holes 241, 242 as shown. If the stator caps 220, 230 are of
zirconia and the body is of a plastic composite, balancing holes 241, 242
would need to be plugged with zirconia weights of appropriate size.
Precision tolerances are required so that the inner stator caps 220, 230,
inner bearing sleeve 210, and dynamic balancing holes and weights are
located with radial repeatability of less than d.sub.2 /200 for zirconia
or d.sub.2 /50 for composites, but errors less than half that large are
preferred. Press-fits are unacceptable for convenience reasons. Precision
slip-fits--i.e., positive mean radial clearances less than 8 microns for
composites, and less than 4 microns for ceramics--are preferred. The inner
stator caps and sleeve are secured by the bearing races 202, 203, which in
turn are secured by the outer turbine blisks 261, 262. The outer turbine
blisks are internally threaded with a fine, precision thread 263, 264 to
match the external, axial threads on the ends of the outer rotor body 201.
The turbine blisk and bearing race could be joined as a single part, but
this is not usually preferable for manufacturing reasons, especially since
different materials would usually be preferred for the race and blisk.
Radial-inflow turbine blades 265, 266 are located on the ends of the outer
turbine blisks along with outer-rotor thrust bearing annuli 267, 268 for
rotation and centering. Disassembly is accomplished by unscrewing an outer
turbine blisk, slipping the bearing race off, pulling a stator cap out,
removing the inner rotor, and unscrewing an inner turbine cap.
The end of the outer rotor is shown in FIG. 3b, where turbine blades 266
may be seen.
Most of the features of the outer stator as depicted in FIG. 1 are very
similar to the prior-art supersonic MAS design, as disclosed in co-pending
application PCT/US91/01225. The most significant difference is the
requirement of providing pressurized gas through axial injectors 11, 12
into inlet ports 251, 252. This gas may come from the outer-stator bearing
supply 21. Also, the rf magnetization coil 31 will usually be somewhat
shorter than in MAS owing to the reduced sample dimensions in DOR.
Performance at high frequencies may be enhanced somewhat by the addition
of copper rf shield-rings 32, 33, shown in FIG. 1, to sharply limit the rf
magnetic field beyond the sample region. B.sub.1 homogeneity is adversely
affected, but Q is greatly enhanced--especially when the outer-rotor body
is made from a carbon-filled composite. However, it will often be
desirable to replace the DOR outer-rotor with a conventional, cylindrical
MAS rotor, according to the prior art, in which case the rf-flux-shorting
rings are undesirable.
The low thermal expansion of carbon-fiber-reinforced plastics facilitates
operation over a wide temperature range with a combination of zirconia and
composites. Temperatures up to 165.degree. C., for example, are possible
with carbon-filled PEK for all the inner-rotor, outer-rotor, and
outer-stator parts, although the inner bearing sleeve and outer bearing
races would generally be of zirconia for wear resistance. Higher
temperature operation--up to 250.degree. C.--is possible with a zirconia
sample cell while the rest of the parts are still of composites. For even
higher temperatures, the entire DOR spinner may be made from zirconia.
The low-carbon composites do not have sufficient strength for sample cells
for high-density samples even at room temperature unless excessively thick
walls are used in the sample cell. FIG. 4 shows an inner rotor design
appropriate for a zirconia sample cell 401 with composite inner turbine
caps 402, 403. To maintain the proper moment ratio and interchangeability
with composite inner-rotors without using zirconia turbine caps, the
outside diameter of the zirconia sample cell 401 must be reduced over the
central region. For .sup.17 O studies on high-density samples, sample cell
401 could be made from silicon nitride with a somewhat larger central
diameter. For very high temperatures, it becomes necessary to utilize
zirconia or silicon nitride turbine caps even though their precision
internal threads are very difficult to grind and gage. A zirconia sample
cell design according to FIGS. 2a and 2b may be used with zirconia turbine
caps. An alternative all-zirconia, two-part sample cell is shown in FIG. 5
that is much easier to manufacture to the required precision (couple
imbalance is easily made negligible), but it is more likely to unscrew
during instability.
Although this invention has been described herein with reference to
specific embodiments, it will be recognized that changes and modifications
may be made without departing from the spirit of the present invention.
All such modifications and changes are intended to be included within the
scope of the following claims.
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